The invention concerns a light barrier or a light grid having a light emitter and a spaced-apart light receiver with an alignment beam between them for mutually aligning the emitter and receiver.
Such light barriers or light grids are used to monitor access zones, for example, in the vicinity of dangerous machine tools.
A light barrier has a single light emitter and a single light receiver which protect the access zone with a light beam that crosses the access zone. A light grid has several light emitters and receivers that generate several light beams which are set off and parallel to each other and cross the access zone. In this application the term “light barrier” is used to refer to both a light beam and a light grid unless otherwise indicated.
Further, the word “light” is not confined to visible light and is intended to generally refer to those electromagnetic rays, from UV light through the visible range to IR light, that are normally used for the operation of light barriers and light grids.
The light barriers described herein use the single-path principle. The light emitter (for light barriers) or emitters (for light grids) are installed in a first housing and the light receiver or receivers in a second housing. In order for the light emitters on one side of the monitored access zone to properly cooperate with the light receivers on the other side of the access zone, it is necessary to mutually align the two housings with respect to each other. For this, they must be linearly shifted, rotated or tilted, as needed.
It is known to use alignment aids for aligning these housings. Especially when the distance between light emitter and receiver is great, the housings can be aligned with a permanently installed or an externally positioned, adaptable sighting telescope. It is also known to align the emitter and receiver with the help of the actual light signal of the light barrier. In this method, the orientation of the light emitter and the light receiver is changed until the received signal has reached a maximum value. For this, it is known, for example, to vary the blinking frequency of a display diode as a function of the strength of the reception signal, or to generate an acoustic signal, for example.
Another mechanism for aligning the two housings is known from published European patent application EP 0 889 332 B1. In this publication, a method and a device are disclosed in which an alignment light emitter is located in each housing for sending out an alignment light beam. Alternatively, a second alignment beam can be generated from the first alignment beam with several, precisely adjusted deflection mirrors.
Drawbacks in the known method of aligning light barriers are the cost associated with the sighting telescope solution and the space which it requires. In the solution making use of the received signal beam, the time that is required for aligning the emitter and receiver is quite high.
A drawback for the alignment aid disclosed in EP 0 889 332 B1 is that the mutual geometrical orientation of the two housings necessitates that the opposite housing always be visible to carry out the alignment. This can lead to problems, especially when the distances are large and there are high levels of relatively bright ambient light. This type of alignment aid is especially difficult when the light barriers are guided by deflection mirrors through an angular access zone. Furthermore, such an alignment aid is relatively expensive due to the fact that an alignment light emitter has to be installed and operated in each housing, or because several exactly adjusted deflection mirrors are required. Moreover, much time is needed to achieve the desired alignment.